The occurrence of cyclical, orbital motion in cylindrical bodies is a phenomenon well known in the art of rotating machinery. Such motion is commonly the product of the existence of an imbalance condition in a wheel, shaft, or other like component which results in a wobbling motion having a frequency equal to that of the angular speed of the shaft.
Even the most finely balanced rotating machinery is not always free of this condition, taking for example the central shaft of a gas turbine engine. Although normally operating without significant vibration, this central shaft is occasionally subject to temporary thermal bowing as a result of being held horizontally in a non-moving, rest position for a sufficient length of time. Upon startup of a previously inert engine wherein the central shaft has experienced such bowing, the engine will experience a temporary imbalance condition until such time as the shaft restores itself to operating condition.
As will be appreciated by those skilled in the art of rotating machinery, the existence of an imbalance in a rotating member results in a greatly increased demand on the bearing components to restrain the movement of the rotating member or shaft and to transfer the lateral forces induced by the imbalance into the machinery mounting structure. This increased demand is especially undesirable in high performance gas turbine engines wherein it is preferable to design the shaft bearings and bearing supports so as to minimize rotating friction while accommodating normal bearing loads rather than to provide a bearing which, although resistive to the temporary startup rotor imbalance condition, is far stiffer and heavier than required during normal engine operation.
One method of reducing these lateral and other stresses on the shaft bearings in a gas turbine engine is by the use of a fluid sgueeze damper between the outer portion of the central shaft bearing race and the supporting engine case. The damper is a hydrodynamic system wherein a continuously flowing stream of damping fluid, such as oil, is supplied to an annular volume formed between the non-rotating outer bearing race and the engine support case for the purpose of absorbing and reducing the transverse movement induced by shaft imbalance, temporary or otherwise.
The flowing oil, typically supplied from the engine lubricating system, fills the annular volume and exits through a vent opening typically placed at the top of the annular volume. The vented fluid is allowed to drain into a scavenge sump or the like in the engine from which it is recycled back to the engine and damper by means of a fluid pump.
Fluid squeeze dampers of the prior art as just described have proved effective in absorbing the transverse orbital movement of a gas turbine engine shaft under limited imbalance conditions. Under conditions of extreme shaft wobbling induced by rapid startup of a bowed shaft, prior art dampers are subject to both cavitation of the damping fluid in the annular volume as well as the influx of air through the vent opening, due to the occurrence of local pressures within the annulus that are lower than ambient. The presence of a separated gas component in the annulus significantly reduces damper effectiveness.
The prior art custom of dealing with such damper cavitation and dryout has been to restrict engine startup timing during the initial imbalance period, a program which is less desirable to those engine users who require immediate engine operability, such as the military.